This invention concerns sewage treatment processes and systems, and particularly involves a method and system used in a plant having membrane bioreactors for intensifying a treatment process, sometimes lowering air scour requirements through superoxygenation of raw wastewater feed (influent) or mixed liquor in the MBR tank, while providing needed oxygen for the intensified process. The method of oxygenating the influent can also break down refractory organics, improving process efficiency.
Membrane air scouring is necessary to remove the solids that accumulate during filtration and generally accounts for 25%-75% of total overall system energy demand. Due to inefficient diffusers and oxygen transfer rate (OTR) limitations, much of the oxygen contained in scouring air is wasted to the atmosphere instead of being used for biological processes. Moreover, increasing mixed liquor suspended solids (MLSS) concentration for the purposes of reducing process volumes further reduces OTR, requiring systems to be larger and more complicated to operate. This invention uses a stream of supersaturated influent and/or mixed liquor to provide all or most of the biological process oxygen, while also in most cases reducing the amount of air required for membrane scouring purposes. This invention reduces the need for diffused aeration. The term “supersaturate” with oxygen as used herein means to introduce more oxygen into solution than normally can be retained in the liquid if in an open tank at ambient conditions, i.e. atmospheric pressure (which is about 10 mg/l).
Most submerged MBR technologies use diffused aeration to scour away materials that accumulate on membrane surfaces during filtration (so-called jet aeration is an alternative). Generally, coarse bubble aeration has been shown to be the most efficient means of air scouring but it is the least efficient means of delivering oxygen to the biological process. Given increasing energy costs, some manufacturers have turned to fine-bubble diffusers for better oxygen delivery and to provide air scouring, with attendant maintenance issues and increased equipment costs. Regardless of the aeration technology used to provide scouring air, the oxygen transfer rate (OTR) limits how much a biological process can be intensified (volume reduced) as diffuser performance drops off precipitously with increasing mixed liquor suspended solids concentrations (MLSS).
Prior oxygenation systems have been proposed that introduce (pure) oxygen into mixed liquor through agitation or mixing. However, these technologies introduce oxygen into other zones in the biological flow sheet and not directly into the influent or the MBR tank, and none has suggested utilizing the supersaturated oxygen to meet some of the membrane air scour requirements.
Due to increased energy costs, it would be greatly advantageous if supersaturation of oxygen could be used in an MBR system for process enhancement through increased oxygen content and also, in most cases, for scouring aeration to thereby reduce separate air scour requirements.